1. Field of the Invention
The present invention relates to a tracking error detecting circuit which is applied to a data recording and reproducing apparatus for recording and/or reproducing data with respect to a disc-shaped optical recording medium, for example, an optical disc, magneto-optical disc, or phase change optical disc, and detecting the tracking error for tracking on the disc-shaped recording medium.
2. Description of the Related Art
In a recording and reproducing apparatus for performing recording and/or reproduction with respect to a so-called optical disc such as a compact disc (CD) or Mini Disc (MD), the main tracking servo method used for suitably following a track has been the three-spot system of comparing the amounts of the reflected light from two sub-beams arranged sandwiching a main beam.
From the viewpoints of the simplification of the apparatus, reduction of size, and reliability, however, attention has been paid to the "push-pull" method with which a tracking error can be detected by a single spot. The push-pull method is a method which makes use of the fact that the distribution of the intensity of light refracted and reflected by a pit or a groove and striking an object lens again changes according to the relative position of the spot with the pit or groove. It receives the reflected light at a photo detector divided into a plurality of sections and finds the tracking error based on the difference of the amount of the light received at the sections of the photo detector.
In this push-pull method, however, it suffers from the disadvantage in that, when the object lens moves, the spot moves on the photo detector and a DC offset is caused in the tracking error signal. This DC offset is caused when use is made of a pick-up having a configuration where only the object lens moves or where the disc surface is inclined 90 degrees relative to the light axis of the beam etc. Therefore, where the push-pull method is used, it is necessary to cancel this DC offset.
Where tracking is performed with respect disc in which a "wobble" groove is formed, the method enabling the tracking error to be suitably detected by cancelling such a DC offset that is frequently used is the wobble push-pull (WPP) method.
This WPP method is a method effective where the tracking servo is applied to a disc-shaped recording medium in which a wobble groove (meandering track) is formed in the track and utilizes the fact that the amplitude of the wobble frequency component contained in the output signal of the photo detector changes according to the position of the object lens. Namely, the amplitude of the wobble frequency component contained in the light detection signal is detected, the position of the object lens is found, and the offset value caused in the tracking error is cancelled.
The tracking error signal TE detected by this WPP method is represented by for example following Equation 1. EQU TE=(E-F)/(E+F)-K.sub.w x (Ew-Fw)/(E-F)w (1)
where,
E is first order diffraction light, PA1 F is minus (-) first order diffraction light, and PA1 Ew, Fw, and (E-F)w are wobble amplitude components of E, F, and (E-F).
In Equation 1, the first term on the right side corresponds to the push-pull signal, and the second term corresponds to the offset signal to be cancelled. Further, the coefficient K.sub.w for finding the offset signal to be cancelled is set to a predetermined value in advance based on the characteristics of the disc of the recording medium, characteristics of the optical pick-up, characteristics of the circuit, etc. and after further considering also the variation of them at production.
In this way, the coefficient K.sub.w for finding the offset cancellation value is set based on the average characteristics of the produced plurality of recording media and recording and reproducing apparatuses so that it can also handle variations of the recording media and recording and reproducing apparatuses. However, this means that, when viewing the individual recording and reproducing apparatuses and recording media mounted in the recording and reproducing apparatuses, the coefficient K.sub.w optimum for the tracking conditions thereof has not been determined. For this reason, there has arisen a demand that the offset cancellation value be sought with a higher precision by using a more suitable coefficient K.sub.w.
Further, it is considered that various characteristics of the recording and reproducing apparatuses and the recording media for determining the coefficient K.sub.w gradually change along with use or along with the elapse of time, for example, aging of the characteristics of the circuit and deterioration of the laser output of the optical pick-up. For this reason, it also suffers from the disadvantage that even a coefficient K.sub.w which was substantially suitable when the recording and reproducing apparatus and recording medium were produced will deviate along with the elapse of time and it will become impossible to find an adequate offset value.
The detecting circuit of such a tracking error signal is constituted while being integrated in an RF-IC for processing the RF signal from the head in many cases. The coefficient K.sub.w as mentioned before could not be easily changed.
Further, due to the rise of recording densities and the rise of access speeds in recent years, a higher precision of tracking has been required. For this reason, it has been demanded that the offset value caused in the tracking error signal be correctly found and suitably cancelled and a correct tracking error signal be found.
Further, the offset component cancellation amount sought by for example K.sub.w .times.(Ew-Fw)/(E-F)w based on the wobble component changes in the amount detected according to the direction of lens shift so it suffers from the disadvantage that a difference from the offset amount produced in an actual push-pull signal PP(E-F)(E+F) is caused due to this, the offset amount cannot be correctly found, and, as a result, a correct tracking error signal cannot be obtained.
The dependency of the push-pull signal indicated by (E-F)/(E+F) with respect to the direction of the lens shift becomes symmetrical with respect to the lens shift 0 as indicated by d in FIG. 1. Namely, in FIG. 1, y1 is almost equal to y2. No matter in which direction of the lens shifts, an amount of offset not according to that direction, but depending upon the amount of shift thereof is produced.
On the other hand, the wobble component obtained from the light detection signal has a characteristic as shown in FIG. 2. Curve a in FIG. 2 shows the dependency of the wobble component Ew/(E-F)w corresponding to the first term of the offset component as indicated as Kw.times.(Ew-Fw)(E-F)W upon the lens shift; and curve e shows the dependency of the wobble component (Fw/(E-F)w corresponding to the second term thereof upon the lens shift. When the offset component is found as indicated by Kw.times.(Ew-Fw)/(E-F)w from two signals having different dependencies with respect to the lens shift, a signal is given where the offset component is dependent on the direction of the lens shift as shown in FIG. 3. Namely, in FIG. 3, y3&gt;&gt;y4. Even if the shift amount is the same, a large difference is caused in the offset component calculated according to the direction of the lens shift.
As apparent from a comparison of FIG. 1 and FIG. 3 in this way, where a lens shift is carried out in a minus(-) direction in FIG. 1 and FIG. 3, the offset component actually produced in the push-pull signal shown in FIG. 1 and an offset component calculated by Kw.times.(Ew-Fw)/(E-F)w shown in FIG. 3 are greatly different, that is, a correct offset component cannot be calculated in that case. As a result, there arises a problem in that the correct tracking error signal cannot be obtained.